News Feature
| March 10, 2014

D.C. Installs Two New Sewage Vats

Washington D.C. recently installed two new vats at the Blue Plains Treatment Plant.

The plant, operated by D.C. Water, is "the biggest facility of its kind on Earth," the Washington City Paper reported. It has handled the city's waste for 73 years.

Along with installing the new vats, D.C. Water is also building a major underground sewage pathway using a massive underground tunnel boring machine named "Lady Bird."

In a recent article, Washington Post provided a close-up look at the new vats and how they function.

The vats, which have already been built, are referred to as the surge or screening shaft and the dewatering pump station, the Post said. One of the vats serves as the entry point for the city's new sewage tunnel, which remains under construction.

The vats are large, concrete, side-by-side cylinders. The screening shaft is 81 feet around. The pump state is 139 feet around, the report said.

What does the screening shaft do ? "Sewage flushing down a tunnel 23 feet in diameter can carry some large stuff with it. For example, shopping carts and the carcasses of large animals are not uncommon. The screening shaft catches those in a two-inch coarse screen. Then it passes the rest of the sewage onto the dewatering pump, where it goes through a quarter-inch fine screen," the report said.

As for the dewatering pump, it uses something called "a ballasted clarification process" to screen particles out of the water. Using chemicals and sand, the pump prompts materials to fall to the bottom of the tank. That's where they are removed. "Grit also is removed using centrifugal force units.The last step is disinfection with bleach to kill microorganisms followed by dechlorination," the report explained.

Blue Plains "treats an average of more than 300 million gallons per day of wastewater from the District, Maryland, and Virginia suburbs, as well as millions of gallons of runoff from city streets," according to Potomac River Guide.